完蛋(wán dàn) - to be done for, to be finished in Mandarin

完蛋 (wán dàn) is the spoken register's way of saying that something is beyond saving: a person in trouble, a plan wrecked, a machine dead. English has "done for", "had it" and "finished", and 完蛋 sits comfortably with all three.

It is not a transparent compound. The egg contributes nothing you could work out from the parts. 蛋 turns up in a whole family of blunt colloquialisms about people and outcomes, and here it is simply the sound the slang wanted.

The trap is register. 完蛋 is casual and a little coarse, fine among friends and wrong in a meeting or an email. In writing use 失败 (to fail) or 结束 (to end). And note it takes 了 almost obligatorily: 完蛋了, not bare 完蛋, because the point is that the change has already happened.